Cell Relay Archive

["Will Fookes" <will@noSpam.wfookes.co.uk>]

I wonder if anybody can provide some guidance...

I wish to set up an SVC for CBR video transmission with a known bitrate,
using AAL1 with forward error correction.  This is easy, my SETUP message
contains (inter alia) the optional AAL Parameters IE as described in Q.2931
with the appropriate AAL1 fields completed:
   AAL type = AAL1
   AAL1 Subtype = video signal transport
   CBR rate = video bitrate / multiplier
   Multiplier = 64 kbit/s
   Clock recovery = adaptive
   Error correction = FEC (loss sensitive)

The SETUP message also contains an ATM Traffic Descriptor IE with a Forward
Peak Cell Rate (CLP = 0+1).

Quite why it is necessary to duplicate the rate information in these two IEs
eludes me (though I can accept that you don't _have_ to match your cell rate
to the video bitrate).

But what should I do if the SVC is to carry backwards traffic as well as
forwards? Particularly when the bitrate and cellrate are different in each
direction? Does the AAL IE indicate the forwards bitrate, or the backwards
bitrate?  Clearly I have to use the same AAL in each direction, so does this
imply that everything about the directions must be symmetric (in which case
my forwards-only SVC is illegal)?

The situation is worse when I wish to set up an SVC for backwards-only flow
(ie. receive-only). Under UNI 4.0 I can send the Minimum Acceptable Traffic
Descriptor (if the network supports it), allowing the far end to pick a
suitable cellrate. It makes little sense for the call originator to specify
the bitrate in this case.

I suppose I'm hoping that somebody can tell me that networks never use the
AAL1 CBR specified in the AAL IE, and that provided I put something in there
(zero?) the IE will always be passed through to the far end to interpret?

Will Fookes